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Abstract

We demonstrate optical time-domain spectroscopy from femtoseconds to nanoseconds using an ultrafast dual-fiber-laser system with kilohertz continuous scanning rates. Utilizing different wavelengths for the pump and probe beams, we exploit this system’s broad range of timescales for quantitative studies of thermal transport and the detection of coherent spin and lattice excitations in epitaxial magnetic thin films. The extraordinary temporal dynamic range provides a way to connect the fast and slow timescales in the observation of dissipation and decoherence processes.

The heat diffusion model (equations 2–7 from [15]) can be applied if the time constant of heat diffusion in film (τf) and interface time constant (τi) follows τf/τi=d σk/kf<1 (equation 10), where d is the film thickness. If σk~108 to 109 W/m2K and kf is from few tens to few hundreds W/Km then d should be <100 nm, which verifies that d for our samples (70 nm) satisfies this criterion.

The heat diffusion model (equations 2–7 from [15]) can be applied if the time constant of heat diffusion in film (τf) and interface time constant (τi) follows τf/τi=d σk/kf<1 (equation 10), where d is the film thickness. If σk~108 to 109 W/m2K and kf is from few tens to few hundreds W/Km then d should be <100 nm, which verifies that d for our samples (70 nm) satisfies this criterion.

Fig. 2. Coherent optical phonon reflectivity oscillations detected in an Sb thin film grown on (111) Si substrate. Pump (1560 nm) and probe (789 nm) beams are both s-polarized and collinear. FFT in the inset corresponds to the oscillatory part of the signal after subtraction of slowly varying background.

Fig. 3. Scheme used for component-resolved MOKE separation described in the text. Vertical (s-polarized) and horizontal (p-polarized) lines represent the incident probe polarization on the sample. Sample-induced MOKE polarization rotation, for longitudinal and polar magnetization components, is sketched using displacement of arrows. Dashed lines at 45° represent the orientation axis of the analyzer placed in the probe beam after reflection on the sample.

Fig. 5. Experimental transient reflectivity in the upper curve (two lower curves) is measured using 780 nm (1560 nm) pump beam wavelength. The curves are rescaled and displaced for clarity. FFT in the inset corresponds to the oscillatory part of the signal of experimental curves for Au/Fe/Ge (100) and Au/Fe/Ge (110).

Fig. 6. Coherent magnetization oscillation measurement for (110) Fe/Ge sample. Oscillatory trace detected at 780 nm probe wavelength, and H≈1000 Oe, is shown in (a), the lower dotted (red line) curve is the experiment (fit) after background subtraction; (b) equivalent magnetic field linewidth values are plotted as dots (see text) and the line is a fit; (c) shows a comparison of the experimental results obtained at 780 nm and 520 nm probe wavelengths, and H≈100 Oe, in the upper and lower curves, respectively. ΔI/I is the fractional transmission through analyzer in a) and c).